Filth one suffers in prison (7)
I believe the answer is:
dungeon
'prison' is the definition.
(a dungeon is a prison cell)
'filth one suffers in' is the wordplay.
'filth' becomes 'dung' (excrement).
'suffers' is an anagram indicator.
'in' indicates putting letters inside.
'one' anagrammed gives 'eon'.
'dung' placed within 'eon' is 'DUNGEON'.
(Other definitions for dungeon that I've seen before include "Keep" , "Underground prison-cell" , "Prison cell, often underground" , "old prison" , "Place of confinement underground" .)